CD4 T cells are a crucial component of the adaptive immune system to provide immunological protection to infectious pathogens. Although much is understood about the mechanisms of CD4 differentiation in vitro, how the heterogeneity of CD4 T cell responses is generated and maintained during infection in vivo is unknown, and will be the central focus of this proposal. First, we will establish when the cel fate decisions to diferentiate into distinct CD4 T cell subsets are determined by barcoding and tracing individual nave CD4 T cels. Although we know that CD4 T cell responses are heterogeneous, we do not understand when this choice is made during infection in vivo. Secondly we will address whether inflammatory signals such as IFN- and IL-6 play reciprocal roles in the differentiation of distinct TH1 subsets during LCMV infection. Additionally, we will determine if direct IL-6 signaling plays a role in TH1 memory development during influenza virus infection, and finally we will ask whether Spi2A plays a similar role in protecting memory CD4 T cell from cathepsin-mediated cell death as it does in CD8 T cells. Immunological memory is essential for protection and the experiments proposed will greatly aide in our understanding of how CD4 T cell responses are generated and maintained to memory. Thus, this work wil help to define the mechanisms involved anti-viral CD4 T cell differentiation and memory development in order to develop more eficacious vaccines.